The objectives of this project are to study those factors which regulate cyclic AMP and cyclic GMP phosphodiesterase activities. Three classes of cyclic nucleotide phosphodiesterases have been partially purified from rat liver and HTC hepatoma cells. E-I exhibits classical Michaelis Menten kinetics and a high affinity for cGMP. E-I is activated by Ca ions plus a heat-stable protein, by chymotrypsin, and by a heat-labile protein from renel cortex. Treatment with dexamethasone reduces the activity of E-I, and does not alter the activity of the heat stable protein. E-II exhibits positively cooperative kinetics, is inactivated by chymotrypsin and not altered by the renal activator. Interaction of E-II with cGMP leads to enhanced susceptibility to chymotryptic inactivation and thermal inactivation. Treatment with dexamethasone leads to a decrease in the activity of E-II. E-III exhibits a high affinity for cAMP and is activated by the renal activator. Incubation of HTC cells with dibutyryl cAMP leads to a 2-fold increase in E-III activity with no change in either E-I or E-II activity or the activity of the heat stable protein activator. Work reported under Project #ZO1 HL 00608-03 CM has been incorporated into this project. BIBLIOGRAPHIC REFERENCES: Lovell-Smith, C.J., Manganiello, V.C., and Vaughan, M.: Solubilization and characterization of hormone-responsive phosphodiesterase activity of rat fat cells. Biochim. Biophys. Acta 497:447-458, 1977. Ross, P.S., Manganiello, V.C., and Vaughan, M.: Regulation of cyclic nucleotide phosphodiesterases in cultured hepatoma cells by dexamethasone and N6, O2 -dibutyryl adenosine 3':5'-monophosphate. J. Biol. Chem. 252:1448-1452, 1977.